1. Field of the Invention
The present invention relates to switching networks in general and more particularly to switching networks that connect data transmission systems with communications media.
2. Prior Art
In the past, numerous types of communications systems have been used to facilitate the exchange of data between machines. The ring or loop communications system is one of the well-known types. Although the loop system can be arranged in many different configurations, the basic configuration usually includes a transmission medium formed into a ring or closed loop. The transmission medium may be coaxial cables, copper wires, fiber optics, etc. The transmission medium is usually prewired into a building.
In order to gain access to the transmission medium, a plurality of devices called concentrators are provided. The concentrators are connected via relatively short lengths of transmission medium to respective wall plugs. By attaching a data terminal equipment to the wall plugs, a user is able to transfer information out of the ring to a recipient. An example of the prior art ring communications system is described in an article entitled. "IBM 8100 Information System, Communications and Loop Description," Form No. GA27-2883.
Each concentrator is provided with one or more switching devices. The function of the switching device is to insert an attached data terminal equipment into the ring and/or to remove the DTE from the ring. If a DTE is inserted, electrical signals are fed from the loop into the DTE. The DTE may use the signal or it may be retransmitted onto the transmission medium. Similarly, if the DTE is disconnected, it is bypassed by signals on the loop.
In the past electromechanical relays have been used as the chief device for switching the DTE into and/or out of the loop. Relays often work well for their intended purpose. However, the relays are plagued with several problems. Among the many problems are the following: unreliability, expensive to manufacture and very difficult to switch. The switching problem is augmented when it must be done remotely over long cable lengths. U.S. Pat. Nos. 4,417,242; 4,255,741; 4,435,704; 3,633,166 and 3,458,665 are examples of prior art devices in which some form of electromechanical relays aid in coupling or decoupling a station to a transmission medium.